The present invention relates to an electrophotographic color image forming apparatus in which a plurality of imagewise exposure means and a plurality of developing means are arranged along the circumferential surface of the image forming member, and toner images are superimposed on the surface so that a color image is formed while the image forming member makes one turn.
There have been known the following apparatuses as a method of forming a multi-color image:
an apparatus (A) in which photoreceptor drums, charging devices and developing devices each corresponding in number to colors necessary for forming the multi-color image are provided, and a mono-color toner image formed on each photoreceptor drum is superimposed one after another on an intermediate transfer member so as to form a color image,
an apparatus (B) in which a single photoreceptor drum makes plural turns so that the charging, imagewise exposure and developing processes for each color are repeated so as to form a color image, and
an apparatus (C) in which the charging, imagewise exposure and developing processes are conducted for each color sequentially while a single photoreceptor drum makes one turn so as to form a color image.
However, the apparatus (A) has a drawback that a size of the apparatus become too large because the plural photoreceptor drums and the intermediate transfer members are required. The dimensions of the apparatus (B) can be made relatively smaller because the required number of each of the charging means, imagewise exposure means and photoreceptor is only a single, however, the apparatus (B) has a limitation that the size of a formed image is limited not to be larger than the area of the circumferential surface of the photoreceptor drum.
For the aforegoing, apparatus (C) does not need plural photoreceptors and intermediate transfer objects, and thereby the apparatus can be made very small, and a size of an image is not restricted by the surface area of the photoreceptor, thereby large-sized images can be formed. Moreover, it is possible to carry out high speed image forming.
However, it is ncessary to provide plural sets of charging units, imagewise exposure means and developing units within a circumferential surface of the photoreceptor, and it is also necessary to superpose with high accuracy the toner images each having a different color formed by plural sets of imagewise exposure means and developing units. In particular, accuracy with which the plural toner images are superposed is affected by accuracy of imagewise exposure timing of plural imagewise exposure means on the photoreceptor. For example, position deviation of plural imagewise exposure means caused by temperature change, or position deviation of plural imagewise exposure means caused by inclination or non-linearity of imagewise exposure means such as, for example, LED array taking place in assembling, may cause a lag in the aforesaid imagewise exposure timing.
In addition, the imagewise exposure timing itself varies when an angular velocity of the photoreceptor changes. For example, when control is made to obtain constant peripheral speed, the angular velocity of the photoreceptor is changed by variation (rariation caused by temperature change or variation caused by replacement of a photoreceptor) of an outer diameter of the photoreceptor. As a result, color doubling is caused in superposing images each having a different color.
When the color doubling is caused, images each having a different color are superposed imperfectly, and the so-called color doubling is caused, resulting in inability of obtaining color images with high image quality.
The invention is to solve the problems mentioned above, and its object is to provide a color image forming apparatus wherein color doubling in superposed images made by plural imagewise exposure means on a photoreceptor can be adjusted automatically, and thereby each toner image can be superposed on the photoreceptor accurately for forming color images with high image quality.
In the case of a monochromatic image forming apparatus wherein an LED array composed of point light source arranged straight to be a straight line form, for example, is used as an imagewise exposure means mentioned above, when an amount of image deformation on a transfer material caused by tilt, a bend and doubling of a mounting position of the imagewise exposure means is small (approximately 0.1 mm), this deformation is not observed visually, causing no problem in practical use. Even in the case of the aforesaid apparatus (B) that is a color image forming apparatus of a type of collective transfer by multi-rotation, the deformation of each color image has been the same, causing no doubling in super position, because the same imagewise exposure means has been used for forming images each being different in color. However, in the case of apparatus (C) that is a color image forming apparatus when a plurality of imagewise exposure means are used and an image is formed within one turn through collective transfer, doubling and blotting are caused by tilt, skew and doubling of a mounting position of the imagewise exposure means, became imagewise exposure means each bring different for each color are used. Doubling and blotting tend to be observed visually (about 0.03 mm), causing a problem in practical use.
Namely, in the color image forming apparatus of apparatus (C), when arranged LED arrays are not uniform in linearity and directions, doubling is caused on superposed toner images, causing doubling and blotting which deteriorates a color image badly.
Problems stated above have been solved for improvement in the invention, and its second object is to provide color image forming apparatus (c) and a control method therefor wherein LED arrays are grouped in the primary scanning direction to be controlled in terms of exposure timing so that the LED arrays are corrected to be uniform in linearity and directions for exposure.
After solving the probems mentioned above to obtain improvement, the third object of the invention is to provide a color image forming apparatus wherein imagewise exposure timing for each of plural imagewise exposure means can be adjusted automatically depending on variation of a length of the circumferential surface of an image forming object and variation of exposure positions of LED arrays, and thereby toner images each having a different color can be superposed correctly to form a color image of high image quality.